loon vs birdbrain

loon and birdbrain both are nouns.

Word	Noun	Adjective	Verb	Adverb
loon	Yes	No	No	No
birdbrain	Yes	No	No	No

As nouns, loon and birdbrain are synonyms defined as:

loon and birdbrain: a person with confused ideas; incapable of serious thought

Other synonyms of loon include addle-head, addlehead.

loon (noun)	birdbrain (noun)
large somewhat primitive fish-eating diving bird of the Northern Hemisphere having webbed feet placed far back; related to the grebes	a person with confused ideas; incapable of serious thought
a worthless lazy fellow
a person with confused ideas; incapable of serious thought